Heavy Metal Pumps in Plants: Structure, Function and Origin

In all forms of life, primary active transport of heavy metal ions ismediated bymembers of a subfamily of P-type ATPase pumps, the P1B ATPases. All organisms require heavy metals such as copper and zinc for essential life processes, but too much heavy metal is hazardous for cells, so control of cellular heavy metal homeostasis is essential. Heavy metal exporting P1B ATPases evolved in prokaryotes where they are omnipresent. In this review, we discuss the structure, function and evolution of plant P1B ATPases. These pumps control heavy metal homeostasis in the plant both at the cellular and organismal levels. They operate by a mechanism similar to other P-type ATPase but have structural features that make them specific for copper or zinc, although a subset of pumps have less specificity. Remarkably, zinc pumps are absent from animals, which employ secondary active transport systems for the export of zinc. A subset of copper pumps in plants share an evolutionary origin with animal copper pumps, but chloroplastic copper pumps evolved from endosymbiotic gene transfer from cyanobacteria. In plants and other photosynthetic eukaryotes, two types of zinc pumps may both have a cyanobacterial origin.